Truly: Greek, a·menʹ, a transliteration of the Hebrew ʼa·menʹ, meaning “so be it,” or “surely.” Jesus frequently uses this expression to preface a statement, a promise, or a prophecy, thereby emphasizing its absolute truthfulness and reliability. Jesus’ use of “truly,” or amen, in this way is said to be unique in sacred literature. When repeated in succession (a·menʹ a·menʹ), as is the case throughout the Gospel of John, Jesus’ expression is translated “most truly.”​—See study note on Joh 1:51.

sooner would heaven and earth pass away: Hyperbole that is equivalent to “never.” The Scriptures indicate that the literal heavens and earth will remain forever.​—Ps 78:69; 119:90.

smallest letter: In the Hebrew alphabet current at that time, the smallest letter was yod (י).

one stroke of a letter: Certain Hebrew characters featured a tiny stroke that differentiated one letter from another. Jesus’ hyperbole thus emphasized that God’s Word would be fulfilled down to the smallest detail.